In March 2003, arsenic was detected from the drinking water from wells in Kamisu city, Ibaraki Prefecture, at a concentration as high as 450-fold the water quality standard. Investigations made thereafter revealed that this arsenic originated from one species of organoarsenic compound, specifically, a phenylated arsenic compound containing diphenylarsinic acid [(C6H5)2AsO(OH); the structural formula is shown as formula (I)] as a main component.

Contamination by this phenylated arsenic compound directly resulted in damages to the health of neighboring residents, and incorporation of arsenic into the rice produced in the vicinal paddy fields was also detected. The arsenic contamination of the soil or underground water in this area, which has been caused by diphenylarsinic acid, seems to be still in progress to a large extent, and thus investigations involving monitoring by the Ministry of the Environment of Japan are being continuously carried out to the present.
Diphenylarsinic acid is a raw material for the synthesis of diphenylcyanoarsine or diphenylchloroarsine, which are both used as emetics (sternutators), which belong to a kind of chemical weapons, and diphenylarsinic acid is also a hydrolysis product. When diphenylcyanoarsine or diphenylchloroarsine is dumped into the soil, the compounds undergo conversion to diphenylarsine hydroxide and bis(diphenylarsine) oxide, and are finally converted to diphenylarsinic acid. In Japan, the soil contamination which is believed to be caused by abandonment of chemical weapons including these organoarsenic compounds, is being discovered at the sites where military installations constructed 60 or more years ago used to be present, or the neighborhood. Also, in abroad countries, organoarsenic contamination of soil or underground water which is believed to be caused by the dumping of chemical weapons, is being confirmed. However, technologies for the restoration of such organoarsenic-contaminated soil are yet to be established.
As a conventional method for purifying soils contaminated with organoarsenic compounds, a method of extracting and removing organoarsenic compounds from the contaminated soil by washing treatments has been proposed (Patent Document 1). In that case, any of an aqueous solution containing sodium hydroxide, an aqueous solution containing phosphoric acid or a salt thereof, an aqueous solution containing sulfuric acid, an aqueous solution containing hydrochloric acid, an aqueous solution containing tartaric acid or a salt thereof, an aqueous solution containing citric acid or a salt thereof, and an aqueous solution containing oxalic acid or a salt thereof, is used as the washing agent.
As a method for purifying organoarsenic compound-containing water, there has been a proposed method of treating an organoarsenic compound by reacting the organoarsenic compound with hydrogen peroxide in the presence of at least one metal ion selected from the group consisting of iron ions, copper ions, cobalt ions and manganese ions, to thereby oxidatively degrading the organoarsenic compound to inorganic arsenic (Patent Document 2). Alternatively, there has been a proposed method of reducing an organoarsenic compound by adding an inorganic flocculant such as ferric chloride and an organic polymer flocculant to form flocs, precipitating and separating these flocs, subsequently filtering the precipitation treatment water through a sand filter, and then further subjecting the treated water to adsorption with activated carbon (Patent Document 3). There has also been a proposed method characterized by degrading an organoarsenic compound to an inorganic arsenic compound by blowing ozone gas while irradiating the organoarsenic compound-containing water with ultraviolet radiation (Patent Document 4).
Furthermore, there have been a proposed water treatment method characterized in that a precipitation process of adding a flocculant to water containing organic arsenic to precipitate and remove the contained organic arsenic is carried out, followed by a reverse osmosis membrane process of removing any organic arsenic remaining in the water by means of a reverse osmosis membrane; and a water treatment apparatus including a reverse osmosis membrane apparatus for treating a water containing organic arsenic, the water treatment apparatus being characterized by having a precipitation tank for precipitating the organic arsenic by mixing a flocculant to the water, which is disposed upstream to the reverse osmosis membrane apparatus (Patent Document 5).
However, in the case of applying the above-described method for purifying organoarsenic compounds to any contaminated soil, it is necessary to perform excavation of contaminated soil or water abstraction, extraction of organic arsenic, and the like, and thus enormous efforts and costs are required. Therefore, a method which is more convenient and capable of in situ purification is desired, and as one of the new environment purifying technologies appropriate for such requirements, bioremediation is attracting the public attention. For the implementation of the technology, a microorganism capable of effectively degrading and removing the subject contaminant is indispensable, but in the case of applying this technique to the environmental contamination involving organoarsenic compounds, there is a problem that no useful microorganism has been isolated.
To date, as for the microorganisms degrading organoarsenic compounds, strain K8, strain 12M17 and strain 7M5 have been reported as microorganisms capable of degrading dimethylarsinic acid, and strain K-1′ and strain IV-1 have been reported as microorganisms capable of degrading monomethylarsonic acid (Patent Document 6). The strain K-1′ and strain IV-1 are reported to be capable of degrading phenylarsonic acid as well, albeit only slightly (Patent Document 6). However, there is no bacterium known hitherto as degrading diphenylarsinic acid, which is an organoarsenic compound detected in highest concentrations with regard to the arsenic contamination in Kamisu city as mentioned above.
The disclosures of Patent Document 1 (Japanese Patent Application Laid-Open (JP-A) No. 2005-169162), Patent Document 2 (JP-A No. 2001-158622), Patent Document 3 (JP-A No. 2005-238184), Patent Document 4 (JP-A No. 2005-334761), Patent Document 5 (JP-A No. 2006-43616) and Patent Document 6 (JP-A No. 2005-229945) are incorporated as part of the description of the present specification.    Patent Document 1: JP-A No. 2005-169162    Patent Document 2: JP-A No. 2001-158622    Patent Document 3: JP-A No. 2005-238184    Patent Document 4: JP-A No. 2005-334761    Patent Document 5: JP-A No. 2006-43616    Patent Document 6: JP-A No. 2005-229945